/*
 * Copyright 2002-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.reactive.result.view.script;

import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;
import java.util.Locale;
import java.util.Map;
import java.util.function.Function;

import javax.script.Invocable;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import javax.script.SimpleBindings;

import reactor.core.publisher.Mono;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanFactoryUtils;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextException;
import org.springframework.context.i18n.LocaleContextHolder;
import org.springframework.core.io.Resource;
import org.springframework.http.MediaType;
import org.springframework.lang.Nullable;
import org.springframework.scripting.support.StandardScriptEvalException;
import org.springframework.scripting.support.StandardScriptUtils;
import org.springframework.util.Assert;
import org.springframework.util.FileCopyUtils;
import org.springframework.util.ObjectUtils;
import org.springframework.util.StringUtils;
import org.springframework.web.reactive.result.view.AbstractUrlBasedView;
import org.springframework.web.server.ServerWebExchange;

/**
 * An {@link AbstractUrlBasedView} subclass designed to run any template library
 * based on a JSR-223 script engine.
 *
 * <p>If not set, each property is auto-detected by looking up a single
 * {@link ScriptTemplateConfig} bean in the web application context and using
 * it to obtain the configured properties.
 *
 * <p>The Nashorn JavaScript engine requires Java 8+ and may require setting the
 * {@code sharedEngine} property to {@code false} in order to run properly. See
 * {@link ScriptTemplateConfigurer#setSharedEngine(Boolean)} for more details.
 *
 * @author Sebastien Deleuze
 * @author Juergen Hoeller
 * @see ScriptTemplateConfigurer
 * @see ScriptTemplateViewResolver
 * @since 5.0
 */
public class ScriptTemplateView extends AbstractUrlBasedView {

    private static final String DEFAULT_RESOURCE_LOADER_PATH = "classpath:";


    @Nullable
    private ScriptEngine engine;

    @Nullable
    private String engineName;

    @Nullable
    private Boolean sharedEngine;

    @Nullable
    private String[] scripts;

    @Nullable
    private String renderObject;

    @Nullable
    private String renderFunction;

    @Nullable
    private String[] resourceLoaderPaths;

    @Nullable
    private volatile ScriptEngineManager scriptEngineManager;


    /**
     * Constructor for use as a bean.
     *
     * @see #setUrl
     */
    public ScriptTemplateView() {
    }

    /**
     * Create a new ScriptTemplateView with the given URL.
     */
    public ScriptTemplateView(String url) {
        super(url);
    }

    /**
     * See {@link ScriptTemplateConfigurer#setEngineName(String)} documentation.
     */
    public void setEngineName(String engineName) {
        this.engineName = engineName;
    }

    /**
     * See {@link ScriptTemplateConfigurer#setSharedEngine(Boolean)} documentation.
     */
    public void setSharedEngine(Boolean sharedEngine) {
        this.sharedEngine = sharedEngine;
    }

    /**
     * See {@link ScriptTemplateConfigurer#setScripts(String...)} documentation.
     */
    public void setScripts(String... scripts) {
        this.scripts = scripts;
    }

    /**
     * See {@link ScriptTemplateConfigurer#setRenderObject(String)} documentation.
     */
    public void setRenderObject(String renderObject) {
        this.renderObject = renderObject;
    }

    /**
     * See {@link ScriptTemplateConfigurer#setRenderFunction(String)} documentation.
     */
    public void setRenderFunction(String functionName) {
        this.renderFunction = functionName;
    }

    /**
     * See {@link ScriptTemplateConfigurer#setResourceLoaderPath(String)} documentation.
     */
    public void setResourceLoaderPath(String resourceLoaderPath) {
        String[] paths = StringUtils.commaDelimitedListToStringArray(resourceLoaderPath);
        this.resourceLoaderPaths = new String[paths.length + 1];
        this.resourceLoaderPaths[0] = "";
        for (int i = 0; i < paths.length; i++) {
            String path = paths[i];
            if (!path.endsWith("/") && !path.endsWith(":")) {
                path = path + "/";
            }
            this.resourceLoaderPaths[i + 1] = path;
        }
    }

    @Override
    public void setApplicationContext(@Nullable ApplicationContext context) {
        super.setApplicationContext(context);

        ScriptTemplateConfig viewConfig = autodetectViewConfig();
        if (this.engine == null && viewConfig.getEngine() != null) {
            setEngine(viewConfig.getEngine());
        }
        if (this.engineName == null && viewConfig.getEngineName() != null) {
            this.engineName = viewConfig.getEngineName();
        }
        if (this.scripts == null && viewConfig.getScripts() != null) {
            this.scripts = viewConfig.getScripts();
        }
        if (this.renderObject == null && viewConfig.getRenderObject() != null) {
            this.renderObject = viewConfig.getRenderObject();
        }
        if (this.renderFunction == null && viewConfig.getRenderFunction() != null) {
            this.renderFunction = viewConfig.getRenderFunction();
        }
        if (viewConfig.getCharset() != null) {
            setDefaultCharset(viewConfig.getCharset());
        }
        if (this.resourceLoaderPaths == null) {
            String resourceLoaderPath = viewConfig.getResourceLoaderPath();
            setResourceLoaderPath(resourceLoaderPath != null ? resourceLoaderPath : DEFAULT_RESOURCE_LOADER_PATH);
        }
        if (this.sharedEngine == null && viewConfig.isSharedEngine() != null) {
            this.sharedEngine = viewConfig.isSharedEngine();
        }

        Assert.isTrue(!(this.engine != null && this.engineName != null),
                "You should define either 'engine' or 'engineName', not both.");
        Assert.isTrue(!(this.engine == null && this.engineName == null),
                "No script engine found, please specify either 'engine' or 'engineName'.");

        if (Boolean.FALSE.equals(this.sharedEngine)) {
            Assert.isTrue(this.engineName != null,
                    "When 'sharedEngine' is set to false, you should specify the " +
                            "script engine using the 'engineName' property, not the 'engine' one.");
        }
        else if (this.engine != null) {
            loadScripts(this.engine);
        }
        else {
            setEngine(createEngineFromName(this.engineName));
        }

        if (this.renderFunction != null && this.engine != null) {
            Assert.isInstanceOf(Invocable.class, this.engine,
                    "ScriptEngine must implement Invocable when 'renderFunction' is specified");
        }
    }

    protected ScriptEngine getEngine() {
        if (Boolean.FALSE.equals(this.sharedEngine)) {
            Assert.state(this.engineName != null, "No engine name specified");
            return createEngineFromName(this.engineName);
        }
        else {
            Assert.state(this.engine != null, "No shared engine available");
            return this.engine;
        }
    }

    /**
     * See {@link ScriptTemplateConfigurer#setEngine(ScriptEngine)} documentation.
     */
    public void setEngine(ScriptEngine engine) {
        this.engine = engine;
    }

    protected ScriptEngine createEngineFromName(String engineName) {
        ScriptEngineManager scriptEngineManager = this.scriptEngineManager;
        if (scriptEngineManager == null) {
            scriptEngineManager = new ScriptEngineManager(obtainApplicationContext().getClassLoader());
            this.scriptEngineManager = scriptEngineManager;
        }

        ScriptEngine engine = StandardScriptUtils.retrieveEngineByName(scriptEngineManager, engineName);
        loadScripts(engine);
        return engine;
    }

    protected void loadScripts(ScriptEngine engine) {
        if (!ObjectUtils.isEmpty(this.scripts)) {
            for (String script : this.scripts) {
                Resource resource = getResource(script);
                if (resource == null) {
                    throw new IllegalStateException("Script resource [" + script + "] not found");
                }
                try {
                    engine.eval(new InputStreamReader(resource.getInputStream()));
                }
                catch (Throwable ex) {
                    throw new IllegalStateException("Failed to evaluate script [" + script + "]", ex);
                }
            }
        }
    }

    @Nullable
    protected Resource getResource(String location) {
        if (this.resourceLoaderPaths != null) {
            for (String path : this.resourceLoaderPaths) {
                Resource resource = obtainApplicationContext().getResource(path + location);
                if (resource.exists()) {
                    return resource;
                }
            }
        }
        return null;
    }

    protected ScriptTemplateConfig autodetectViewConfig() throws BeansException {
        try {
            return BeanFactoryUtils.beanOfTypeIncludingAncestors(
                    obtainApplicationContext(), ScriptTemplateConfig.class, true, false);
        }
        catch (NoSuchBeanDefinitionException ex) {
            throw new ApplicationContextException("Expected a single ScriptTemplateConfig bean in the current " +
                    "web application context or the parent root context: ScriptTemplateConfigurer is " +
                    "the usual implementation. This bean may have any name.", ex);
        }
    }

    @Override
    public boolean checkResourceExists(Locale locale) throws Exception {
        String url = getUrl();
        Assert.state(url != null, "'url' not set");
        return (getResource(url) != null);
    }

    @Override
    protected Mono<Void> renderInternal(
            Map<String, Object> model, @Nullable MediaType contentType, ServerWebExchange exchange) {

        return exchange.getResponse().writeWith(Mono.fromCallable(() -> {
            try {
                ScriptEngine engine = getEngine();
                String url = getUrl();
                Assert.state(url != null, "'url' not set");
                String template = getTemplate(url);

                Function<String, String> templateLoader = path -> {
                    try {
                        return getTemplate(path);
                    }
                    catch (IOException ex) {
                        throw new IllegalStateException(ex);
                    }
                };

                Locale locale = LocaleContextHolder.getLocale(exchange.getLocaleContext());
                RenderingContext context = new RenderingContext(
                        obtainApplicationContext(), locale, templateLoader, url);

                Object html;
                if (this.renderFunction == null) {
                    SimpleBindings bindings = new SimpleBindings();
                    bindings.putAll(model);
                    model.put("renderingContext", context);
                    html = engine.eval(template, bindings);
                }
                else if (this.renderObject != null) {
                    Object thiz = engine.eval(this.renderObject);
                    html = ((Invocable) engine).invokeMethod(thiz, this.renderFunction, template, model, context);
                }
                else {
                    html = ((Invocable) engine).invokeFunction(this.renderFunction, template, model, context);
                }

                byte[] bytes = String.valueOf(html).getBytes(StandardCharsets.UTF_8);
                return exchange.getResponse().bufferFactory().wrap(bytes); // just wrapping, no allocation
            }
            catch (ScriptException ex) {
                throw new IllegalStateException("Failed to render script template", new StandardScriptEvalException(ex));
            }
            catch (Exception ex) {
                throw new IllegalStateException("Failed to render script template", ex);
            }
        }));
    }

    protected String getTemplate(String path) throws IOException {
        Resource resource = getResource(path);
        if (resource == null) {
            throw new IllegalStateException("Template resource [" + path + "] not found");
        }
        InputStreamReader reader = new InputStreamReader(resource.getInputStream(), getDefaultCharset());
        return FileCopyUtils.copyToString(reader);
    }

}
